1. Field of the Invention
The present invention generally relates to a driving force control apparatus for a vehicle in which a pair of drive wheels are driven by a drive torque from an electric motor. Preferably, the electric motor is driven with electric power generated by a generator that is driven by an internal combustion engine. The present invention especially useful in an all wheel drive vehicle in which a pair of main drive wheels are driven by a main drive source, such as internal combustion engine, and a pair of subordinate drive wheels are driven by the electric motor. Thus, the invention is particularly well-suited for a so-called battery-less four-wheel drive vehicle in which the engine drives the generator and the electric power from the generator is supplied to the electric motor.
2. Background Information
One example of a conventional vehicle driving force control apparatus in which a pair of front wheels are driven with an engine, a pair of rear wheels are configured such that they can be driven with an electric motor, and a clutch and a reduction gear are installed in the torque transmission path between the motor and the rear wheel axles is presented in Japanese Laid-Open Patent Publication No. 11-243604. This publication discloses a technology in which when the vehicle shifts into a four-wheel drive state while traveling. The clutch is connected after the motor has been rotated in an unloaded state until the rotational speed of the motor equals a rotational speed corresponding to the rotational speed of the wheel axles. As a result, the occurrence of shock is avoided when the clutch is connected. This publication discloses shifting a vehicle into a four-wheel drive state such that the rear wheels are driven by an electric motor when a speed difference equal to or exceeding a prescribed value occurs between the front and rear wheels. Meanwhile, driving of the rear wheels by the electric motor is stopped when the depression amount of the accelerator pedal becomes equal to or less than a prescribed value or the vehicle speed becomes equal to or greater than a prescribed speed.
Another example of a conventional vehicle driving force control apparatus is disclosed in Japanese Laid-Open Utility Model Publication No. 55-138129 (see, page 1, FIG. 2). In this vehicle driving force control apparatus, either the front wheels or the rear wheels are treated as main drive wheels that are driven by power delivered from an internal combustion engine through a transmission, while the rear wheels or front wheels (whichever are not driven by the internal combustion engine) are driven by an electric motor. The electric motor is driven with electric power generated by a generator. The generator is driven by a driving force from the internal combustion engine. When the accelerator pedal is depressed and the vehicle speed is less than or equal to a prescribed value, the electric motor operates such that both the front wheels and rear wheels are driven.
Another example of a conventional vehicle driving force control apparatus is disclosed in Japanese Laid-Open Patent Publication No. 2000-318472. This publication discloses a vehicle driving force control apparatus for a battery-less four-wheel drive vehicle. In this vehicle driving force control apparatus, when the vehicle is traveling at or below a prescribed speed, e.g., 15 km/h, and the speed difference between the front and rear wheels reaches or exceeds a prescribed value, i.e., the acceleration slippage amount reaches or exceeds a prescribed value, a clutch is connected between the motor and the subordinate drive wheels. This clutch connection shifts the vehicle into a four-wheel drive state in which the motor is merely driven or is driven in accordance with the accelerator position. When the accelerator position is at or above a prescribed value, a fixed portion of the engine output is used to drive the generator and the electric power generated with the portion of the engine output being fed to the motor, which drives the subordinate drive wheels. Moreover, when it is time for the vehicle to shift from the four-wheel drive state to the two-wheel drive state, the motor torque command value (target motor torque) is typically reduced and the clutch is released when the target motor torque reaches or falls below a prescribed value.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved driving force control apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.